Non-volatile memories are important elements of integrated circuits due to their ability to maintain data absent a power supply. Phase change materials have been investigated for use in non-volatile memory cells. Phase change memory cells include phase change materials, such as chalcogenide alloys, which are capable of stably transitioning between amorphous and crystalline phases. Each phase exhibits a particular resistance state and the resistance states distinguish the logic values of the memory cell. Specifically, an amorphous state exhibits a relatively high resistance, and a crystalline state exhibits a relatively low resistance.
A typical phase change cell has a layer of phase change material between first and second electrodes. As an example, the phase change material is a chalcogenide alloy, such as Ge2Sb2Te5 or SbTeAg. See, e.g., Lankhorst et al., Low-cost and nanoscale non-volatile memory conceptforfuture silicon chips, NATURE MATERIALS, vol. 4 pp. 347-352 (April 2005).
A portion of the phase change material is set to a particular resistance state according to the amount of current applied via the electrodes. To obtain an amorphous state, a relatively high write current pulse (a reset pulse) is applied through the phase change cell to melt a portion of the material for a short period of time. The current is removed and the cell cools rapidly to a temperature below the glass transition temperature, which results in the portion of the material having an amorphous phase. To obtain a crystalline state, a lower current write pulse (a set pulse) is applied to the phase change cell for a longer period of time to heat the material to a temperature below its melting point. This causes the amorphous portion of the material to re-crystallize to a crystalline phase that is maintained once the current is removed and the cell is rapidly cooled.
A sought after characteristic of non-volatile memory is low power consumption. Often, however, phase change memory cells require large operating currents. It is therefore desirable to provide a phase change memory cell with reduced current requirements. For phase change memory cells, it is necessary to have a current density that will heat the phase change material past its melting point and quench it in an amorphous state. One way to increase current density is to decrease the size of a bottom electrode; another way is to deposit small conductive crystals on the bottom electrode. These methods maximize the current density at the bottom electrode interface to the phase change material.
It would be desirable, however, to maximize the current density at a location above the bottom electrode in certain applications.